Enthalpies can be added based on the property of additivity. This means that the enthalpy change for a reaction can be calculated by adding up the enthalpy changes for the individual steps of the reaction. For example, the enthalpy change for the combustion of methane can be calculated by adding up the enthalpy changes for the following steps:

1. The breaking of the C-H bonds in methane

2. The formation of C-O bonds in carbon dioxide

3. The formation of H-O bonds in water

The additivity of enthalpies is a very useful property because it allows us to calculate the enthalpy change for a reaction without having to measure it directly. This can be especially useful for reactions that are difficult or impossible to measure directly, such as reactions that occur in the atmosphere or in outer space.